Infrared Spectroscopy of Zn(Acetylene)_n⁺ Complexes: Ligand Activation and Nascent Polymerization

Abstract

Zinc–acetylene ion–molecule complexes were produced by laser vaporization in a supersonic expansion. These complexes were mass selected and studied with infrared laser photodissociation spectroscopy complemented by computational chemistry. The combined approach of infrared spectroscopy and theory provides information on the structures and bonding of these complexes, as well as evidence for intracluster reactions. Fragmentation patterns demonstrate that the coordination number of strongly bonded ligands is three. Infrared spectra compared to those predicted by theory allow identification of different isomers at each cluster size. The coordination in these complexes varies between η2 and η1 metal-acetylene connections. Structures based on η2 bonding form a symmetric D 3h configuration for the n = 3 complex. This unreactive core ion forms larger clusters with only weakly bonded acetylene in solvation structures. Structures based on three η1-bonded acetylenes form a near-C 3v core ion which is the doorway configuration for subsequent reactions. Electron transfer to the next (fourth) acetylene produces a metal–carbon bond and a trans-bent metal–vinyl structure with a terminal radical site. This radical site attaches a fifth acetylene to produce a vinyl-dimer structure. Evidence for continued reactions in the larger clusters is obscured by solvating acetylenes with more intense IR bands. The asymmetric coordination of zinc cations and the critical configuration with three-fold coordination that leads to reactivity are new features of intracluster metal–molecular reactions.

Coordination and Solvation in Gas-Phase Ag+(C2H2)n Complexes Studied with Selected-Ion Infrared Spectroscopy

“…Our group has previously used infrared spectroscopy to investigate cation−π interactions in metal cation–acetylene and −ethylene complexes. − These studies on size-selected complexes have allowed us to systematically investigate the bonding interactions of coordinating and solvating ligands. The bonding in these systems is explained by using the Dewar–Chatt–Duncanson − complexation model.…”

“…Additionally, studies on vanadium cations with multiple acetylene ligands identified the key metallacycle intermediates as well as the reaction product in the cyclotrimerization of acetylene to form benzene . Most recently, we documented the asymmetric coordination, ligand activation, and polymerization of acetylene in complexes with a zinc cation . Previously, Krossing and co-workers studied the structures and bonding interactions of Ag + (C 2 H 2 ) n ( n = 1, 3, 4) complexes in solid-state salts with weakly coordinating anions .…”

Cyclotrimerization of Acetylene under Thermal Conditions: Gas-Phase Kinetics of V+ and Fe+ + C2H2

“…Acetylene has the potential to become a major feedstock chemical for industrial purposes. , The mechanism of polymerization/oligomerization of acetylene is also of interest in soot formation and formation of interstellar polyaromatic compounds . Additional interest comes from work that shows that the association of acetylene to transition metal cations results in intracluster reactions that form benzene and possibly other larger unsaturated hydrocarbons. − Cyclotrimerization of acetylene has been specifically pointed out as a catalytic goal in the synthesis of larger molecules. , The reaction of C 2 H 2 + with C 2 H 2 was one of the first ion–molecule reactions studied in the gas phase, and numerous subsequent studies encompassing various experimental and theoretical techniques have been made. − Transition metal cation reactions with C 2 H 2 have followed ,, due, in large part, to the development of the laser vaporization supersonic expansion ion source, , which allows the generation of various metal cations in the gas phase for interrogation by mass spectrometry, laser spectroscopy, and now selected-ion flow tube (SIFT) kinetics.…”

“…Several transition metals have been identified as reactive with acetylene to form clusters with prominent intensity in mass spectra. ,,,− This prominence does not necessarily correlate to stability. Fundamentally, it indicates only that the production mechanisms for that peak occur much more rapidly than the destruction mechanisms under the particular experimental conditions.…”

“…1,2 The reaction of C 2 H 2 + with C 2 H 2 was one of the first ion−molecule reactions studied in the gas phase, 9 and numerous subsequent studies encompassing various experimental and theoretical techniques have been made. 10−16 Transition metal cation reactions with C 2 H 2 have followed 4,5,8 due, in large part, to the development of the laser vaporization supersonic expansion ion source, 17,18 which allows the generation of various metal cations in the gas phase for interrogation by mass spectrometry, laser spectroscopy, and now selected-ion flow tube 19 (SIFT) kinetics.…”